1. Field of the Invention
The present invention relates to a recording method and more particularly to improvements in the method of making color records by causing droplets of recording liquids to adhere onto recording media.
2. Description of the Prior Art
Ink-jet recording comprises ejecting droplets of recording liquid (ink) by any of various techniques for discharging ink (e.g. electrostatic attraction, vibrating or displacing ink mechanically by using a piezoelectric device, and driving ink with pressure caused by heating the ink) and causing some or all of the ejected droplets to adhere onto a recording medium such as paper. This recording process draws attention as a recording method for being quieter and enabling high-speed multicolor printing.
In ink-jet recording, chiefly aqueous inks have been used as recording liquids in view of safety and printability while common paper generally has been employed as recording medium. When recording is made by using a recording liquid, it is required that the recording liquid does not run on the recording paper so as not to make the printed letters obscure, and it is desirable that the recording liquid becomes dry rapidly after application so as not to stain the recording surface.
In particular, multicolor ink-jet recording employing two or more different color inks needs to satisfy the following various requirements:
(1) Recording liquids must be quickly absorbed by the recording medium, and an ink dot, when overlapping a dot of previously applied ink of different color, must not mix with or disorder or diffuse it. PA1 (2) Recording liquids must diffuse on the recording medium to an extent that does not increase the diameter of the ink dots more than necessary. PA1 (3) The shape of each ink dot must be nearly a true circle and the perimeter thereof must not become obscure. PA1 (4) Ink dots must have high optical density and distinct perimeter lines. PA1 (5) The recording medium must exhibit a high brightness and a good contrast to the ink dots. PA1 (6) The color of the applied recording liquid must not vary depending on the recording medium used. PA1 (7) Dimensions of the recording medium must be changed little by recording (that is, elongation or wrinkling is minimized).
While it has been understood that the satisfaction of these requirements depend on characteristics of the recording medium and liquid used, no ink-jet recording method meeting all of these requirements has yet been found in practice.
For instance, ink-jet recording performance of recording media depends on the water content of the recording liquids is as follows: Wood-free paper for office purposes, when used together with usual recording liquids (water contents of ca. 70%), exhibits poor ink absorbability and gives nonuniform ink spreading and markedly inferior shapes of ink dots. In the case of the same paper with recording liquids of less water content (up to 50%), the ink absorbability is slightly improved but the shapes and colors of the ink dots are inferior. Commercial non-coated paper, although exhibiting sufficient ink absorbability regardless of water content, gives ink dots of inferior shapes and colors. In the case of coated paper for printing used with recording liquids of higher water content (at least 70%), the resulting ink dots are good in shape but low in optical density and the absorbability of the ink is much inferior. With recording liquids of less water content (up to 50%), on the contrary, this type of paper shows better ink absorbability but gives ink dots of still low optical density.
Ink-jet recording performance of recording media depends on the surface tension of the recording liquids as follows: In ink-jet recording on a conventional recording medium having a recording layer (art paper), recording liquids of high surface tension show low rates of absorption since only fine interstices are present at the recording surface and hence the penetration of such a recording liquid into these interstices is retarded. When recording liquids of low surface tension are used to increase rates of ink absorption, the spread of ink on the recording surface increases, similarly to the case with non-coated paper, and good recording results will not be obtained. These problems conflicting each other have not been solved.
Moreover, ink-jet recording performance of recording media depends on the viscosity of the recording liquids as follows: For ink-jet recording on non-coated paper, recording liquids of relatively high viscosities are rather suited; by use of a recording liquid having a viscosity of at least 12 cp, so-called blotting of ink dots due to the ink spreading over the surface of paper can be avoided. However, recording liquids with a viscosity that is too high have poor absorbability; when such a recording liquid is used, a prolonged time will be required for ink fixing, and other undesirable effects may occur such as different-color ink droplets, an overlapping at the recording surface, mix with one another and ink dots are enlarged more than is necessary or becomes disordered.
For the purpose of avoiding such undesirable matters, the use of coated paper has been proposed. However, existing coated paper, on which the blotting of ink dots can be avoided, are suited rather for recording liquids of low viscosities up to 2 cp, since the ink absorbability of coated paper is low. Recording liquids having viscosities exceeding 2 cp are limited in water resistance, discharge stability, and stability of solutions of recording agents therein.
For ink-jet recording, the image to be recorded, i.e. an original image, is generally divided into equal sections (picture elements), each of which is expressed by one or more ink dots to reproduce the original image on a recording medium. In this recording method, it is necessary to obtain a sufficient picture element density (recording density of one picture element as a whole) in order to record an image of adequate density. A way of achieving this is explained below referring to the case, as an example, where each picture element is expressed with one ink dot by using a definite amount of ink having a definite recording agent concentration. In this case, it is desirable that the ink dot printed in each section corresponding to the picture element (hereinafter this section of the recording media is simply referred to as a "picture element") may spread as uniformly as possible over the entire area of the picture element. The reason for this is as follows: when an ink dot much smaller than the area of each picture element is fixed therein, the picture element density is observed as being low even though the recording density of the ink dot itself is high. On the contrary, when a relatively large ink dot is fixed in each picture element, the density of the picture element is high even though the recording density of the ink dot itself is low, since when the ink dot spreads over the entire area of the picture element, the picture element density becomes the maximum value.
This is because the picture element density depends chiefly on the ratio of the ink dot area therein to the blank area.
When recording media having higher ink-spreading ability are used, recording can be carried out by using picture elements larger in each area and therefore the recording speed can be increased.
Accordingly, it is required, in addition to the above noted requirements, for ink-jet recording media to have such proper ink-spreading ability as to spread an applied ink droplet uniformly over the entire area of each picture element having a predetermined size.
For example, common wood-free paper is poor in ink absorbability, and hence sufficiently large ink dots cannot be obtained thereon, shapes of ink dots are very inferior, and moreover no desired uniform picture element density can be obtained. With commercial non-coated paper, the absorbability of inks is sufficient but ink droplets run along fibers of the paper and therefore the degree of ink-spreading is nonuniform on the paper and the shapes and sizes of ink dots are difficult to control and additionally the density of ink dots may vary locally, so that sufficient picture element density cannot be obtained.